Re: [PATCH v7 11/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

[Vlastimil Babka]

On 12/26/24 18:07, Suren Baghdasaryan wrote:
> Introduce functions to increase refcount but with a top limit above
> which they will fail to increase. Setting the limit to 0 indicates
> no limit.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  include/linux/refcount.h | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index 35f039ecb272..e51a49179307 100644
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -137,13 +137,19 @@ static inline unsigned int refcount_read(const refcount_t *r)
>  }
>  
>  static inline __must_check __signed_wrap
> -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> +bool __refcount_add_not_zero_limited(int i, refcount_t *r, int *oldp,
> +				     int limit)
>  {
>  	int old = refcount_read(r);
>  
>  	do {
>  		if (!old)
>  			break;
> +		if (limit && old + i > limit) {

Should this be e.g. "old > limit - i" to avoid overflow and false negative
if someone sets limit close to INT_MAX?

> +			if (oldp)
> +				*oldp = old;
> +			return false;
> +		}
>  	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
>  
>  	if (oldp)
> @@ -155,6 +161,12 @@ bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
>  	return old;
>  }
>  
> +static inline __must_check __signed_wrap
> +bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> +{
> +	return __refcount_add_not_zero_limited(i, r, oldp, 0);
> +}
> +
>  /**
>   * refcount_add_not_zero - add a value to a refcount unless it is 0
>   * @i: the value to add to the refcount
> @@ -213,6 +225,12 @@ static inline void refcount_add(int i, refcount_t *r)
>  	__refcount_add(i, r, NULL);
>  }
>  
> +static inline __must_check bool __refcount_inc_not_zero_limited(refcount_t *r,
> +								int *oldp, int limit)
> +{
> +	return __refcount_add_not_zero_limited(1, r, oldp, limit);
> +}
> +
>  static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
>  {
>  	return __refcount_add_not_zero(1, r, oldp);